1. Field of the Invention
The present invention relates to the field of specimen preparation and more particularly to the preparation of thin specimens for use in transmission electron microscopy.
2. Description of the Related Art
Transmission electron microscopy (TEM) is one of the most important techniques available for the detailed examination and analysis of the micro-structural characteristics of many materials. The TEM technique provides high resolution imaging, sensitive chemical spectroscopy and material analysis of thin specimens with thicknesses in the range of 0.01-0.2 .mu.m. The successful use of TEM in direct observation of fine microstructural features has made the TEM technique a necessary analytical tool in semiconductor circuit fabrication. In the semiconductor industry, the TEM technique is generally used for characterizing the materials to be used in semiconductor circuits and for examining the actual structures fabricated from these materials. It cannot be overemphasized that good specimen preparation is the most important requirement of the transmission electron microscopy technique.
The specimens required for transmission electron microscopy should be very thin to make the specimen transparent to electrons. The prepared specimen should also be representative of that found in the bulk material. In other words the microstructure should be unaltered by the preparation procedure. Particularly, in ultra large scale integrated circuit (ULSI) technologies, cross-sectional TEM specimens are especially important.
In (ULSI) designs, the control of ULSI device parameters requires a detailed knowledge of the cross-sectional geometry of individual devices. This can be provided by the cross-sectional specimens which enable direct imaging to be made of the vertical structure of epitaxial layers, ion implanted layers or device structures in ULSI technologies. A typical cross-sectional specimen preparation involves initially identifying a specific feature to be used for the TEM analysis, such as a particular device in an array of memory cells. After the specific feature has been identified, the wafer is cleaved or sawed to produce a rectangular specimen with this particular device of interest located at the center of this specimen. In the next step, the specimen can be reduced to a rectangle by sawing, grinding and polishing. In general, this final size is defined by the size of the specimen housing of the TEM specimen holder, which generally cannot receive specimens larger than approximately 3 mm in diameter. Following this, the specimen is further thinned down to the electron transparency thickness.
There are various techniques for producing electron transparent specimens for cross-sectional transmission electron microscopy studies. Some of these techniques produce a thin electron transparent region having a wedge profile and thus these techniques may be referred to as wedge forming techniques. One well-known wedge forming technique is ion beam milling. In the ion beam milling technique, the specimen is first glued on a specimen grid and then loaded on a holder and placed in the path of one or more ion beams. The specimen grid is a hollow metallic disk for supporting the specimen during ion milling and the TEM examination. The ion beams, angled with respect to the specimen, gradually remove atoms from the surface of the specimen until a small perforation is formed in the center of the specimen. Due to the gradual thinning occurring towards the perforation, a narrow band of material around the perforation forms a wedge profile which is thin enough to allow high energy electrons to pass through. Although ion milling offers a cross-section with a large wedge, this process takes a very long time to reduce specimen thickness down to the electron transparent range. Additionally, it is difficult to prepare specimens with this process quickly and on a reproducible basis to meet the growing needs of the semiconductor industry for TEM analysis.
Tripod polishers can also be used to mechanically thin the specimens to electron transparency thickness ranges. As is well known in the art of TEM specimen preparation, a tripod polisher is a device for holding specimens on a rotating abrasive medium for the mechanical thinning of the specimens. While the process, a specimen is mounted on the tripod polisher and mechanically polished on one side using a sequence of progressively finer polishing films. When the desired feature is reached on one side, the specimen is flipped over on the tripod polisher and polished from the other side, using the same sequence of polishing films to reach the predefined feature of interest. During the polishing on the second side, the tripod polisher is set at a slight angle to produce a tapered or wedge shaped specimen, with the feature of interest at the thin edge of the specimen. This thin edge is electron transparent, typically having a thickness of approximately 0.1 micron across a typical width of approximately 0.5-1.0 mm. The polishing thus eliminates the need for ion milling or reduces ion milling process time to few minutes. However, specific features can easily be damaged in the tripod polishing process. Furthermore, it is difficult to locate specific features, particularly features with submicron dimensions, using wedge forming techniques.
An alternative to wedge forming techniques is the focused ion beam (FIB) process. A specimen having the feature of interest is initially ground to approximately 50 .mu.m thickness, this thickness being measured transverse to the wafer surface. Commonly, the specimen is dimensioned to have approximately a height of about 0.6 mm (the thickness of the Si wafer) and a 2 mm width, in order to fit on a 3 mm diameter TEM grid. Typically, the feature of interest is half way through the 50 .mu.m thickness of the specimen. A slotted TEM grid is used to support the specimen during the FIB process. However, a quadrant of the supporting grid needs to be cut away to permit access of the ion beam to the specimen surface during the FIB process. The specimen is then centered and glued on a slotted grid with the cross-section of the specimen (the 0.6 mm.times.2 mm surface) onto the grid surface. The wafer surface of the specimen is aligned to face the opening of the grid when it is glued. The grid carrying the specimen is mounted on a FIB sample holder and inserted into a FIB work station. The specimen holder positions the specimen in the correct orientation to allow an ion beam access normal to the specimen surface (originally the wafer surface) where the specific feature is located. The ion beam mills both sides of the feature. The focused ion milling forms a very thin and uniform membrane between two trenches dug by the ion beams. The finished specimen is transferred to the TEM system for imaging and analysis.
However, this FIB milling procedure has several shortcomings, which have thus far limited its application in the TEM specimen preparation field. One problem of this procedure is that the available area for TEM analysis is limited by the size of the membrane, so that only a specific location can be examined. Additionally, trench walls present a particular problem for the TEM x-ray compositional analysis (EDSanalysis), since these walls block the path of x-rays generated in the membrane region. Another problem is associated with the use of the slotted grid on which the specimen is mounted. If the grid is distorted during cutting, the alignment of the specimen surface to the beam becomes very difficult.
Accordingly, a need exists for more reliable specimen preparation techniques. Desirably, such specimen preparation techniques should be compatible with the current semiconductor device fabrication technologies and performance.